Happy Perihelion! (and why winter is shorter than summer)

The Earth reached its closest approach to the Sun late last night, coming within about 91.5 million miles of our life-giving star.

This is about 3 million miles closer to the Sun than our planet will be in July.

It turns out that this is more than just an astronomical oddity. The fact that our orbit around the Sun isn’t quite circular has all sorts of interesting implications for life on Earth, including shorter winters for the Northern Hemisphere.

Let me explain.

Our closest approach to the Sun comes as the tilt of Earth’s axis brings the southern half of the world into a more Sun-facing posture. In the summer the Northern Hemisphere more directly faces the Sun. The day of the maximum tilt facing the Sun is the summer solstice, in June.

Axial tilt. (Earth & Sky)

If the Earth had a circular orbit this tilt would entirely dictate the length of the seasons, but the Earth’s orbit is slightly eccentric, or off-centered.

Based upon the Second Law of Planetary Motion discovered by Johannes Kepler planets move a bit faster when they are closest to the Sun and a bit slower when furthest from the Sun. This is because the Sun’s gravitational attraction (or pull) on the Earth is stronger when the Earth is closest to the Sun.

As a result the Earth is now rushing along at 30.3 kilometers per second (almost 19 miles per second), which is about a kilometer per second faster than it will be traveling in July.

Accordingly, if you actually count up the days of winter — from the winter solstice to the spring equinox — and then do the same with summer you’ll find that the Northern Hemisphere “summer” lasts about five days longer than its winter.